This invention relates to a process for preparing polymers of prochiral olefins, especially vinylaromatic monomers, having a high degree of syndiotacticity. More particularly, the invention relates to such a polymerization process using catalysts comprising certain Group 4 metal complexes in combination with one or more activating cocatalysts, characterized in that the complex comprises a cyclic or noncyclic, non-aromatic, anionic, dienyl ligand group forming a .pi.-bond to the metal. In the complexes, the metal is in the +2 or +3 formal oxidation state.
It is previously known in the art to prepare highly syndiotactic vinyl aromatic polymers and other prochiral olefins by the use of titanium complexes containing a single cyclopentadienyl or substituted cyclopentadienyl group and an activating cocatalyst including alkylalumoxanes, inert, noncoordinating ion forming compounds, Lewis acids and mixtures thereof. Disclosures of such prior art processes are found in U.S. Pat. Nos. 5,045,517, 5,066,741, 5,206,197 and U.S. Pat. No. 5,886,117.
Bis(cyclopentadienyl)titanium complexes in which the titanium is in the +4 formal oxidation state, and olefin polymerization catalysts formed from such by combination with an activating agent, e.g., alumoxane or ammonium borate, are well known in the art. Thus, U.S. Pat. No. 3,242,099 described the formation of olefin polymerization catalysts by the combination of bis(cyclopentadienyl)titanium dihalides with alumoxane. U.S. Pat. No. 5,198,401 disclosed tetravalent bis(cyclopentadienyl)titanium complexes, including at col. 9, lines 24-27, diene-containing complexes and olefin polymerization catalysts obtained by converting such complexes into cationic form. Particularly preferred catalysts are obtained by the combination of ammonium borate salts with the bis(cyclopentadienyl)titanium complexes. Generally, such bis(cyclopentadienyl) containing complexes are inferior for use in catalysts to prepare polymers having a high degree of syndiotacticity.
The preparation of certain Ti, Zr, and Hf monocyclopentadienyl diene complexes was also described in Yamamoto et al., Organometallics, 8, 105 (1989) (Yamamoto) and Blenkers, J, et al., Organometallics, 6, 459 (1987) (Blenkers). Titanium and zirconium complexes in which the titanium is in the +4 formal oxidation state, including monocyclopentadienyl complexes are also known in the art. Examples of the foregoing references include: U.S. Pat. Nos. 5,064,918, 4,774,301, and 4,808,680.
In copending application, Ser. No. 08/082,197 filed Jun. 24, 1993, now abandoned, there are disclosed monocyclopentadienyl complexes having a cyclic structure in which the titanium is in the +2 formal oxidation state. The application also teaches the formation of olefin polymerization catalysts from such complexes by the combination of the complex with activator compounds such as alumoxanes, borate salts and strong Lewis acids. The teachings of all of the foregoing patents, patent applications and publications are hereby incorporated by reference.